1. Technical Field
The present teaching relates to method and system for analog circuits. More specifically, the present teaching relates to method and system for operational amplifiers and systems incorporating the same.
2. Discussion of Technical Background
Operational amplifiers (op-amp) are widely used to drive a load which often corresponds to a capacitive load. A typical op-amp with a Miller feedback is shown as 100 in FIG. 1 (Prior Art). Circuit 100 includes two gain stages 115 and 125 in series (the output of the first gain stage drives the input of the second gain stage) and a Miller feedback element 150. The first gain stage 115 comprises a gm stage (110) with parasitic loads (120 and 121). The first gain stage has a positive voltage gain at zero frequency (DC). A gm stage is a functional block where the output current is a function (normally a linear function) of input voltage difference. The second gain stage 125 comprises a gm stage (130) with an external load (104). The Miller feedback 150 is a capacitive element 160 (sometimes in series with a resistive element 155) from the output of the second gain stage to the output of the first gain stage. The second gain stage 125 has a negative voltage gain at zero frequency (DC).
Specifically, the first gain stage has its positive input connected to the op-amp positive input (180) and its negative input connected to the op-amp negative input (190). At the output of gain stage 115 (VA), a resistor 120 and a capacitor 121 represent the load at that node (including parasitic and next stage load). The second gain stage 125 has its input connected to the first gain stage output (VA). The Miller feedback circuit (150) connects VOUT which is the output of the second gain stage (125) and VA which is the output of the first gain stage (115). Alternatively, the input of the second gain stage (125) may be generated by a buffer (not shown) situated between the output of first gain stage 115 and the input of second gain stage 125.
At the second gain stage, the output of amplifier (VOUT) is connected to the external load (represented by load resistor 135 and a load capacitor 140, which are connected in parallel). The output of the amplifier (VOUT) is also fed back to node VA through the Miller feedback element 150. The Miller feedback element 150 comprises a serially connected resistor 155 and a capacitor 160.
To drive a larger capacitive load, traditionally, a resistor 170 is introduced which is connected in series, with the load capacitor. However, this solution may cause voltage swing problem with degradation of gain bandwidth product and slew rate depending on the load. Therefore, there is a need for an operational amplifier that can drive a larger capacitive load but avoid voltage swing problem while maintaining a good slew rate and a good gain bandwidth product.